The present invention relates to an apparatus for measuring the intake air flow into an internal combustion engine.
Conventionally, as a system for measuring an intake air flow of the internal combustion engine, a hot wire type air flow meter has been widely used. This air flow meter uses a hot wire sensor exposed in an intake air passage. The hot wire is supplied with a current which is controlled to keep constant temperature of the hot wire. An air flow passed through a cross section of the intake air passage, in which the hot wire is disposed, is measured based on the value of the supplied current of the hot wire. This hot wire type air flow meter can measure an absolute value of an air flow, but can not determine the direction of the air flow.
In an internal combustion engine, the intake air pulsates because of the piston reciprocation in the cylinders. The amplitude of the pulsation increases according to the opening angle of the throttle. When the amplitude exceeds a certain value, the intake air flows in the reverse direction temporarily. The reverse flow is called a backflow.
In addition, the hot wire type air flow meter has response delay which is caused by a heat capacity of its own. There is a nonlinear correlation between the air flow and an output of the hot wire type air flow meter.
In control of the internal combustion engine, the mass of the intake airflow is calculated by averaging the pulsating signal from the hot wire type air flow meter. The integrated value of the air flow mass converted from the electric signal, or the integrated value of the electric signal has an error. The error is equal to twice the mass of the backflow.
Several methods have been presented in order to solve the above-described problems.
JP-A-10-300544 discloses a method of compensating the detected signal wherein a threshold value is calculated based on the maximum and minimum values of the detected signal from the hot wire type air flow meter, determining the backflow period in the air flow signal based on comparison of the threshold value and the detected signal, and the detected signal is corrected when a backflow is determined to occur.
JP-A-7-167697 discloses a method for correcting the measured signal from hot wire type air flow meter. In this method, the peak and valley points of the measured signal from a hot wire type air flow meter are detected, a time length from the peak to the valley is calculated, and the existence of the backflow is determined based on the calculated time length. The measured signal is corrected when a backflow is determined to occure.
JP-A-61-213728 discloses a method of correcting the measured signal from hot wire type air flow meter. In this method, a change of direction of the intake air flow is detected by comparing a difference of the measured signal from a hot wire with the threshold value. The backflow is detected based on the changing point of the air flow direction, and the measured signal is corrected when a backflow is determined to occur.
JP-A-2000-265898 discloses a method of correcting the measured signal from hot wire type air flow meter. In this method, a frequency spectrum is extracted from the measured signal, estimating a ratio of a backflow is estimated based on the relationship between a frequency spectrum and a backflow ratio, and the measured signal is corrected by using the backflow.
In a system for detecting intersection whether the detected signal crosses the threshold or not, e.g., in the method described in JP-A-10-300544, the crossing cannot be detected robustly because of the noise in the signal. Thus, there is a problem of reliability in practical use.
In the method described in JP-A-7-167697, the peaks and valleys are detected. However, if noises interfere the measured signal, false maximum and minimum points may detected other than the correct maximum and minimum. Thus, there is a problem of reliability in practical use.
In the method described in JP-A-61-213728, the difference of the measured signal is compared with the threshold. This threshold varies according to the engine condition, such as engine rotation speed and throttle opening angle. It is difficult to find a method for calculating this threshold for all engine condition.
The method described in JP-A-2000-265898 is different from the techniques disclosed in above mentioned three documents, because determination of a backflow section is not performed. This method is advantageous in robustness because a characteristic value, such as a frequency spectrum, is extracted from the entire waveform in a given section of the measured signal, backflow determination is not necessary. However, to extract the frequency spectrum, a revolution speed of the crankshaft must be detected, or estimated from the waveform. To obtain the revolution speed of the crankshaft increases the costs of an input unit, and estimation of the revolution speed from the waveform increases an amount of calculation.
An object of the present invention is to provide an apparatus for measuring an intake air flow of an internal combustion engine, which compensates the backflow effect robustly without determining the flow direction with small calculation cost.
In order to achieve the above-described object, there is provided an apparatus for measuring an intake air flow of an internal combustion engine by using an output of flow detecting means by a heat resistor, comprising: flow converting means for converting the electric signal from the flow detecting means into the mass of the flow; backflow determining means for determining presence of a backflow based on the electric signal outputted by the flow detecting means; kurtosis calculating means for calculating the kurtosis for the distribution of the electric signal from the waveform of the flow detecting means when a backflow is determined to exit; backflow ratio calculating means for calculating the backflow ratio based on the kurtosis; and flow calculating means for integrating the instantaneous mass of the flow converted from the electric signals by the flow converting means, and for calculating the real mass of the flow by compensating the integrated mass with the backflow ratio.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.